Reland "Parallelize fetching telemetry benchmark deps"

This is a reland of 8ab0b856

Original change's description:
> Parallelize fetching telemetry benchmark deps
> 
> The fetching script currently fetches about 2 GB (on my linux workstation)
> from cloud storage serially. This can (and is) trivially parallelizable, and
> should speed up the fetching script significantly.
> 
> This script has been taking 20 minutes on mac machines for a long time. This
> will hopefully significantly speed the mac builders up.
> 
> Bug: 1102597
> Change-Id: I93d2fa2b1f4b4ef6e27e9c894a9dd4c0d0e12d7b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2285503
> Commit-Queue: Stephen Martinis <martiniss@chromium.org>
> Reviewed-by: Wenbin Zhang <wenbinzhang@google.com>
> Reviewed-by: John Chen <johnchen@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#786521}

Bug: 1102597
Change-Id: I37d0055fadb26bf4eae2b0b5fe0ecc4732edde3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2288740
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789736}

tools/perf/fetch_benchmark_deps.py

@@ -7,10 +7,11 @@
 
 import argparse
 import json
+import logging
+import multiprocessing
 import optparse
 import os
 import sys
-import logging
 
 from chrome_telemetry_build import chromium_config
 from core import benchmark_finders
@@ -121,8 +122,12 @@ def main(args):
       raw_input(
           'No benchmark name is specified. Fetching all benchmark deps. '
           'Press enter to continue...')
-    for b in benchmark_finders.GetOfficialBenchmarks():
-      deps[b.Name()] = _FetchDepsForBenchmark(b)
+    benchmarks = benchmark_finders.GetOfficialBenchmarks()
+    p = multiprocessing.Pool()
+    results = p.map(_FetchDepsForBenchmark, benchmarks)
+    p.terminate()
+    for benchmark, result in zip(benchmarks, results):
+      deps[benchmark.Name()] = result
 
   if options.output_deps:
     with open(options.output_deps, 'w') as outfile: